Experiments are proposed to apply pulsed Fourier-transform natural abundance 15 N NMR spectroscopy to characterize nitrogen-containing biological model systems for chemical carcinogens and nucleic acids. The types of information to be gained will be assessed by determination of factors affecting chemical shifts, line shapes, and relaxation times of nitrogen heterocycles, including substituted pyridines, pyrroles, pyrimidines. Because of the possible relationships between electronic properties and biochemical behavior, we expect to use the technique to probe subtle changes in electronic character at nitrogen in alkyl- substituted pyridines and pyrroles. Results are to be applied to characterization of alkaloids and of nucleic acid bases. Evidence for specific complexation at particular sites, as well as for base-stacking interactions will be sought, exploiting the inherently narrow lines of 15 N and the consequent ability to measure small differences in line positions. Relaxation times and nuclear Overhauser effects will be measured on the model compounds, and applied to a series of tobacco alkaloids to determine the nature of any intermolecular association. Substituent parameters in aliphatic systems will be determined in dilute solution in order to separate contributions owing to molecular association from inherent electronic effects on the chemical shifts. Relaxation times will also be used to resolve ambiguities regarding the nature of metal-ion binding to ATP.